It is common practice in the printing industry to protect a signature or a group of signatures from inclement weather and rough handling during their transport to various distribution points. Conventionally groups of stacked and strapped, collated and bound signatures or, in some instances, individual signatures are conveyed to a wrapping machine where the signatures are confined within a transparent or translucent foil of synthetic plastic material for this purpose. The wrapping machines which employ webs of such plastic material are generally designed to loop a single web or strip of plastic material, or to repeatedly unite the leaders of two webs to form a loose loop or envelope for receiving the signatures. The plastic envelope surrounding the signatures is normally cut and sealed by a transversal welding unit after which the packaged signatures are usually conveyed towards an oven where heat shrinking takes place to complete a properly formed, closely sealed package.
There are a limited number of ways commonly accepted for wrapping groups of signatures. In one method, signatures are drawn by a conveyor through a plastic curtain during which a cam driven welding unit supported on a horizontally translatable car is actuated to seal the signatures. Such a method is disclosed in U.S. Pat. No. 4,313,288 issued to Tassi, et al on Feb. 2, 1982, and U.S. Pat. No. 4,831,809 issued to the same inventors on May 23, 1989. U.S. Pat. No. 4,991,376 issued to Backman on Feb. 12, 1991, also discloses a packaging method wherein signatures are conveyed through a plastic curtain. Another method often employed deliberately pushes stacked signatures transversely through a plastic film across a conveyor running generally parallel to the plastic film.
Each of the above methods has drawbacks in applying the proper amount of plastic film for stacks of signatures having variable height. In an attempt to solve this dilemma, various sensing arrangements are utilized at various locations in these packaging machines for selectively metering amounts of plastic film suitably encasing bundles typically ranging in height from less than one inch to nine inches. In addition, a particularly troublesome problem occurs in some machines when individual signatures of low height bundles comprising a relatively small number of stacked signatures are insufficiently stable to traverse the plastic film without distortion. In these cases, the collective flexibility of the signatures is such that the leading edges of the signatures tend to ride upwardly along the plastic film or effect relative movement between the bundle and conveyor and in doing so can compound the problem of proper film payout.
One apparatus which attempts to solve this latter problem is disclosed in aforementioned U.S. Pat. No. 4,991,376. In this arrangement, responsive to the sensing of a particular stacked product, a deflection member for the upper portion of the plastic curtain is displaced so that film is supplied substantially without tensile force, the deflection member being comprised of a movable roller mounted on the end of two vertical cylinder rods. The problem with this apparatus, however, is that plastic film is primarily pulled from an upper roll by the extension of vertical cylinders. Film payout from the lower roll is extracted with the assistance of a suction device on an outfeed conveyor. Without the provision of positively driven plastic film, this apparatus appears dependent upon the stroke length and responsiveness of the cylinders supporting the upper portion of the plastic curtain and it is questionable whether the proper amount of plastic film cam be consistently provided.
From the foregoing, it can be appreciated that an attempt has been made by the prior art to upgrade the wrapping capability for stacked signatures of various heights. However, there remains a need in this well developed art for a system which delivers and packages stacked signatures, especially those of a low height, with a more responsive, positively controlled wrapping function which will more efficiently govern plastic film consumption and minimize bending or buckling of packaged signatures.